Sic crystal and wafer cut from crystal with low dislocation density

ABSTRACT

A method of forming an SiC crystal including placing in an insulated graphite container a seed crystal of SiC, and supporting the seed crystal on a shelf, wherein cushion rings contact the seed crystal on a periphery of top and bottom surfaces of the seed crystal, and where the graphite container does not contact a side surface of the seed crystal; placing a source of Si and C atoms in the insulated graphite container, where the source of Si and C atoms is for transport to the seed crystal to grow the SiC crystal; placing the graphite container in a furnace; heating the furnace; evacuating the furnace; filling the furnace with an inert gas; and maintaining the furnace to support crystal growth to thereby form the SiC crystal.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.14/058,167, filed on Oct. 18, 2013, which claims the benefit of priorityfrom U.S. Provisional Application No. 61/761,171, filed on Feb. 5, 2013,and the disclosures of which are hereby incorporated herein by referencein their entireties.

BACKGROUND

1. Field

The disclosed invention relates to manufacturing of silicon carbide(SiC) crystals and wafers.

2. Related Art

Silicon carbide, SiC, is a crystalline semiconductor material,recognized by those familiar with materials science, electronics andphysics as being advantageous for its wide band gap properties and alsofor extreme hardness, high thermal conductivity and chemical inertproperties. These properties make SiC a very attractive semiconductorfor fabrication of power semiconductor devices, enabling power densityand performance enhancement over devices made from more common materialslike silicon.

The most common forms of SiC consist of cubic or hexagonal arrangementsof atoms. The stacking of Si and C layers can take on many forms, knownas polytypes. The type of silicon carbide crystal is denoted by a numberdenoting the number of repeat units in the stacking sequence followed bya letter representing the crystalline format. For example the 3C—SiCpolytype refers to a repeat unit of 3 and a cubic (C) lattice, while a4H—SiC polytype refers to repeat unit of 4 and a hexagonal (H) lattice.

The different silicon carbide polytypes have some variations inmaterials properties, most notably electrical properties. The 4H—SiCpolytype has the relatively larger bandgap while the 3C—SiC has asmaller bandgap, with the bandgaps for most other polytypes falling inbetween. For high performance power device applications when the bandgapis larger, the material is more capable, in theory, to offer relativelyhigher power and thermal conductivity performance.

SiC crystals do not occur in nature and as such must be synthesized.Growth of SiC crystals can be executed by sublimation/physical vaportransport or chemical vapor deposition.

Growth of SiC by sublimation is very challenging. Temperatures in excessof 2,000° C. are required to generate vapor stream of Si/C species bysublimation, which places great limitations on the reaction cellcomponents and the furnace design. Originally SiC abrasive materialsformed by processes like the Acheson method were used as the source ofthe Si and C atoms for the crystal, and as the technology matured groupsdeveloped means to synthesize SiC source powder specifically for SiCcrystal growth. The growth is usually performed in a graphite containerwithin a vacuum chamber. The graphite container is heated by eitherresistive methods or induction methods. The container is insulated in acareful manner so as to create controlled temperature gradients withinthe volume. A seed crystal is used, which is usually shaped like a plateor disc. The seed crystal is typically oriented with its growth surfacefacing the source material. The location of the seed crystal in thecontainer is designed such that when the container is heated, the seedis at a relatively lower temperature position, while the Si—C sourcematerials are at the higher temperature position. When the container isheated to a temperature sufficient to sublime the source material, thevapors will travel towards the low temperature region and condense onthe seed crystal. While this appears simple in concept, in practice thegrowth of SiC is very complicated and recognized by those who practiceas very difficult to perform.

Historically, initial progress in SiC sublimation-based crystal growthis described first by Lely (U.S. Pat. No. 2,854,364) whose method ofunseeded crystal growth resulted in small hexagonal SiC platelets. Inthe 1970s and 1980s the art to produce the first crystals of sizeattractive for producing devices was done in Russia by Tairov andTsvetkov (Journal of Crystal Growth, 52 (1981) p. 146-50 and Progress inControlling the Growth of Polytypic Crystals in Crystal Growth andCharacterization of Polytype Structures, P. Krishna, ed., PergammonPress, London, p. 111 (1983)). Their approach used a Lely crystal as aseed, and conducted growth by sublimation and transport as describedabove. These results showed methods for polytype control by choice ofseeds, pressure control and temperature gradients. Later, Davis (U.S.Pat. No. 4,866,005) revealed improvements by judicious selection ofsource materials and gradient controls. Refinements on the methods ofTairov, Tsvetkov and Davis continue to be revealed to this day.

When methods to produce larger crystals emerged, focus also moved tocontrol defects in the crystals. Defects can be categorized asinclusions and crystal dislocations. The primary crystalline defects inSiC crystals are screw dislocations. Among these are a special caseknown as a micropipe or hollow core screw dislocations. Additionally,there are basal plane dislocations and threading edge dislocations.These defects originate from many sources. For example, defectscontained in the seed crystal can be passed to the newly grown crystalvolume. Stresses arising from temperature gradients and thermalexpansion mismatch and imparted to the seed and the crystal duringgrowth can result in formation of dislocations. Deviation of thestoichiometry in the sublimation vapor stream from that needed to formSiC can result in unstable polytype growth; in turn leading to polytypeinclusions in the grown crystal, which leads to dislocation formation atthe polytype boundaries. Even interactions between dislocations cancreate or eliminate dislocations.

SiC crystals produced by methods identified have large concentrations ofdislocations. As of this filing, the commonly reported values of screwdislocation and basal plane concentration are nominally5,000-10,000/cm², respectively. The dislocations are most commonlyassessed by sectioning the crystal in the plane normal to the crystalaxis of symmetry. Etching the exposed crystal surface with molten salt,like potassium hydroxide, at temperatures in the 350-500° C. range willreveal the dislocations. Each dislocation type has a unique shape sothey can be uniquely counted. The dislocations are commonly counted andreported as a number divided by the inspection area. Thischaracterization method is useful as it allows for easy correlation ofdefects contained in planar semiconductor devices formed on the crystalplane. There are many examples in the literature which show thatdislocations are not uniformly distributed in the plane of observation.The large count of dislocations makes it very impractical to count everysingle one, especially as today inspections can be required on sectionsgreater than or equal to the equivalent of 100 mm diameter circles. Sothe etched area is sampled to determine the amount of dislocations.Incorrect sampling methods can lead to errors in the estimation of thedislocation concentration associated with larger crystals. In mostreports, the details of the sampling method are not provided, soreplication of reported results can often be difficult, if notimpossible.

Scientists experienced in solid state physics and semiconductor devicesknow that dislocations result in device performance below thetheoretical properties of the material. Therefore, modern effort focusedon improvements of semiconductor SiC crystal quality look to identifyand control the factors which can reduce defects originating in crystalgrowth.

Once large enough crystals are produced, the crystal must be cut andfabricated into wafers in order to be useful to fabricate semiconductordevices using planar fabrication methods. As many semiconductor crystals(e.g., silicon, gallium arsenide) have been successfully developed andcommercialized into wafer products, the methods to fabricate wafers frombulk crystals are known. A review of the common approaches to, andrequirements for wafer fabrication and standard methods ofcharacterization, can be found in Wolf and Tauber, Silicon Processingfor the VLSI Era, Vol. 1—Process Technology, Chapter 1 (LatticePress—1986).

Due to its hardness, fabrication of SiC into wafer substrates presentsunique challenges compared to processing other common semiconductorcrystals like silicon or gallium arsenide. Modifications must be made tothe machines and the choices of abrasives changed beyond commonly usedmaterials. It has been reported that substantial subsurface damage isobservable on mirror polished SiC wafers, and this can be reduced orremoved by using chemical enhanced mechanical polishing methods similarto that used in the silicon industry (Zhou, L., et al., ChemomechanicalPolishing of Silicon Carbide, J. Electrochem. Soc., Vol. 144, no. 6,June 1997, pp. L161-L163).

In order to build semiconductor devices on SiC wafers additionalcrystalline SiC films must be deposited on the wafers to create thedevice active regions with the required conductivity value and conductortype. This is typically done using chemical vapor deposition (CVD)methods. Techniques for growth of SiC by CVD epitaxy have been publishedfrom groups in Russia, Japan and the United States since the 1970's. Themost common chemistry for growth of SiC by CVD is a mixture of a siliconcontaining source gas (e.g., monosilanes or chlorosilanes) and a carboncontaining source gas (e.g., a hydrocarbon gas). A key element to growthof low defect epitaxial layers is that the substrate surface is tiltedaway from the crystal axis of symmetry to allow the chemical atoms toattach to the surface in the stacking order established by the substratecrystal. When the tilt is not adequate, the CVD process will producethree dimensional defects on the surface, and such defects will resultin non-operational semiconductor devices. Surface imperfections, such ascracks, subsurface damage, pits, particles, scratches or contaminationwill interrupt the replication of the wafer's crystal structure by theCVD process (see, for example, Powell and Larkin, Phys. Stat. Sol. (b)202, 529 (1997)). It is important that the polishing and cleaningprocesses used to fabricate the wafer minimize surface imperfections. Inthe presence of these surface imperfections several defects can begenerated in the epitaxial films including basal plane dislocations andcubic SiC inclusions (see for example, Powell, et. al. TransactionsThird International High-Temperature Electronics Conference, Volume 1,pp. II-3-II-8, Sandia National Laboratories, Albuquerque, N. Mex. USA,9-14 Jun. 1996).

Defects in SiC are known to limit or destroy operation of semiconductordevices formed over the defects. Neudeck and Powell reported that hollowcore screw dislocations (micropipes) severely limited voltage blockingperformance in SiC diodes (P. G. Neudeck and J. A. Powell, IEEE ElectronDevice Letters, vol. 15, no. 2, pp. 63-65, (1994)). Neudeck reviewed theimpact of crystal (wafer) and epitaxy originated defects on powerdevices in 1994, highlighting limitations of power device function dueto screw dislocations and morphological epitaxy defects (Neudeck, Mat.Sci. Forum, Vols. 338-342, pp. 1161-1166 (2000)). Hull reported shift tolower values in the distribution of high voltage diode reverse biasleakage current when the diodes were fabricated on substrates havinglower screw dislocation density (Hull, et. al., Mat. Sci. forum, Vol.600-603, p. 931-934 (2009)). Lendenmann reported forward voltagedegradation in bipolar diodes was linked to basal plane dislocations inthe epilayer that originate from basal plane dislocations in thesubstrate (Lendenmann et. al., Mat. Sci. Forum, Vols. 338-342, pp.1161-1166 (2000)).

Modern technology for growth of 4H—SiC crystals has not been successfulto develop a commercial method for a crystal growth process that allowssimultaneous control over the gamut of dislocation types.

Various methods disclosed in the prior art are often lacking in detailsregarding the specific steps employed in the crystal growth or themethods employed to assess the concentration of defects and demonstraterepeatability.

In recent years, it has become desirable to grow larger crystals assilicon carbide and gallium nitride as such materials which are usefulin building high efficiency power/RF diodes and transistor devices.However, large bulk crystals of silicon carbide (SiC) are difficult togrow, in part because such processes require high temperatures of up to2,500° C. Currently, most growth processes utilize vapor-based transportmethods such as physical vapor transport (PVT) sublimation methods.

For example, crystals of SiC may be grown by PVT in a reaction cellwhich is typically formed from solid graphite. The cell is typically aright angle cylinder and the grade of graphite is chosen so that itsthermal expansion coefficient is close to that of SiC in order tominimize stresses imparted by the differences in the respectivecoefficients of expansion. The SiC seed crystal is typically provided ina disk shape. The seed crystal is polished and can be coated on the sideopposite the growth surface with a material which is stable at thegrowth temperatures. The presence of a protective carbon coating canhelp to suppress deterioration of the seed during the crystal growthprocess, as voids (“thermal evaporation cavities”) may form in the seedwhen the protection is absent.

The seed is generally oriented in the reaction cell so that the majoraxis of the cylindrical reaction cell and the plane of the seed waferare nominally at right angles to each other. In most PVT reaction cells,the seed crystal is placed above the Si/C source feedstock to aid incontrolling vapor flow as well as keeping the seed free of debris andcontamination during growth. In this type of PVT reaction cellarrangement, the seed is typically supported above the vapor source byrigid or mechanical attachment of the seed to the lid of the reactioncell. The seed may be attached to the lid by adhesive, cement, retainingrings, and the like. However, the act of mounting the seed to thereaction cell lid or seed holder can lead to undesirable effects duringcrystal growth.

For example, during the mounting process, scratching of the coated seedback may occur. In addition, a void may be created between the seed andreaction cell ceiling interface during the mounting process. Suchoccurrences may exacerbate formation of evaporation cavities of the seedbackside, resulting in defect formation. Moreover, differences inthermal expansion between the seed and lid can cause stresses in theseed. See, for example, discussion in Japanese patent publication JP2011-20860.

It is important to support the seed crystal in the reaction cell in aposition where the temperature can be controlled to allow condensationof the vapor stream on the seed surface. However, because the reactioncell is typically made from a different material than the seed crystal,the rigid attachment methods often create stresses in the crystal duringgrowth. For example, when the seeds are mounted to the reaction cell orlid using an adhesive, it is possible for the seed to be bent during themounting process, which results in undesirable stress in the crystal.

As a result, attempts have been made to develop alternative methods forsupporting the seed crystal during growth. For example, JP 2011-20860relies on pressure differentials to create vacuum that holds the seedagainst the lid. However, contact between the crystal and holder is suchthat stress on the crystal may still occur during growth. Specifically,pressure differential forces may deform the seed. Moreover, if the lid'ssurface is not smooth and flat, it will create pressure points on theback surface of the seed, thereby generating strains in the seed.Similarly, any particulate material that may inadvertently be capturedbetween the seed and the lid would create pressure point. Even if thelid is perfectly flat such that the seed perfectly contacts the lid,when seed touches the lid it will cool the backside of the seed, causingit to bow away from the lid and the seal will be broken. Also, thebowing will cause the crystal to be in stress, leading to defects in thegrowing crystal.

There are other problems with the approach disclosed in the JP2011-20860. For example, when the seed is elevated due to pressuredifferential, it blocks gas flow out of the crucible, thus leading toincreased pressure and temperature inside the crucible. At least forthese reasons it is preferred to maintain the flow of gas out of thecrucible. Also, as the crystal grows it gets heavier and at some pointwill be too heavy for the pressure differential to maintain it againstthe lid, at which point the crystal would drop, leading to sudden changein the temperature of the crystal and due to resumed flow of gas out ofthe crucible, reduced pressure and temperature inside the crucible.

Accordingly, there is a need in the art for a method of SiC crystalgrowth, which results in crystal growth with minimized stress resultingin reduced defects such as micropipes, screw dislocations and basalplane dislocations.

SUMMARY

The following summary of the invention is included in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Aspects of the present invention are directed to a method for supportinga seed crystal during vapor based crystal growth, and more particularly,to a vapor transport growth method wherein the seed crystal is notsubjected to external stress during crystal growth.

Embodiments disclosed herein provide methods to produce a 4H—SiCcrystal/wafer with micropipe density of less than 1/cm², screwdislocation density of less than 5,000/cm² and basal plane dislocationdensity of less than 5,000/cm². All the defect metrics are achievedsimultaneously.

According to disclosed embodiments, polytype 4H—SiC crystals can begrown using seeds of diameter as small as 76 mm or up to and exceeding150 mm diameter.

Various aspects relate to a method of forming an SiC crystal by vaportransport onto a seed crystal, the method comprising: placing a sourceof silicon and carbon atoms in a graphite container, wherein the sourceof silicon and carbon atoms is for transport to the seed crystal to growthe SiC crystal; placing the seed crystal in the graphite container, andsupporting the seed crystal on a shelf within the graphite containerwithout physically attaching the seed crystal to any part of thegraphite container; placing a lid over the container such that the liddoes not contact the seed crystal and preventing the back surface of theseed from contacting the bottom surface of the lid during the entireprocessing cycle, and placing the graphite container in a vacuumfurnace; heating the furnace to a temperature from about 2,000° C. toabout 2,500° C.; evacuating the furnace to a pressure from about 10 Torrto about 100 Torr; and, maintaining the furnace to support crystalgrowth to thereby form the SiC crystal.

According to disclosed aspects a vacuum furnace is maintained to supportcrystal growth to thereby form the SiC crystal having a thickness ofabout 0.1 mm to about 50 mm thick, and wherein nitrogen flow ismaintained such that nitrogen concentration of the grown SiC crystal isfrom about 1×10¹⁵/cm³ to about 1×10¹⁹/cm³.

Disclosed embodiments provide a 4H—SiC substrate cut from a SiC crystaland having an averaged micropipe density of less than about 1/cm², ascrew dislocation density of less than about 5,000/cm², and a basalplane dislocation density of less than about 5,000/cm² as determinedfrom at least 9 measurements made on the 4H—SiC substrate.

Yet further aspects relate to a system for forming an SiC crystal, thesystem comprising: a graphite container having a lid and a shelf forpositioning a seed thereupon; cushion rings allowing the seed to flexand expand while keeping the seed in the desired location relative tothe cell and preventing the seed from contacting the lid during theentire growth cycle; a heater for heating an induction furnace to atemperature from about 2,000° C. to about 2,500° C.; a pump forevacuating the induction furnace to a pressure from about 10 Torr toabout 100 Torr; and gas inlet for filling the induction furnace with aninert gas.

Other aspects relate to a graphite container for use in a furnace forperforming SiC crystal growth on a disk-shaped seed having a knowndiameter, comprising: a graphite cylindrical container having a sidewalland an open top configured for accepting a graphite lid; a cylindricalshelf formed on an upper part of the sidewall and having an interiordiameter slightly smaller than the diameter of the seed, the cylindricalshelf being formed at a defined distance below the open top, therebyenabling placing and supporting the seed thereupon and preventing theseed from contacting the lid during the entire growth process; and agraphite lid configured for forming a closure with the open top.

Additional aspects provide a method for forming an SiC crystal by vaportransport onto a seed crystal, the method comprising: placing a sourceof silicon and carbon atoms in a graphite container, wherein the sourceof silicon and carbon atoms is for transport to the seed crystal to growthe SiC crystal; placing the seed crystal in the graphite container andsupporting the seed crystal such that a maximum stress resulting in theSiC crystal is less than 5×10⁶ dynes/cm2; placing a lid over thecontainer such that the lid does not contact the seed crystal during theentire growth process, and placing the graphite container in a vacuumfurnace; heating the furnace to a temperature from about 2,000° C. toabout 2,500° C.; evacuating the furnace to a pressure from about 10 Torrto about 100 Torr; and, maintaining the furnace to support crystalgrowth to thereby form the SiC crystal.

Other aspects and features of the invention would be apparent from thedetailed description, which is made with reference to the followingdrawings. It should be appreciated that the detailed description and thedrawings provides various non-limiting examples of various embodimentsof the invention, which is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 shows a generic arrangement for growth of SiC crystals byphysical vapor transport, indicative of the prior art.

FIG. 2 shows a physical vapor transport PVT reaction cell, which isconfigured for SiC crystal growth according to one embodiment.

FIG. 3 shows a PVT reaction cell, which is configured for SiC crystalgrowth according to another embodiment.

FIG. 4 shows a PVT reaction cell, which is configured for SiC crystalgrowth according to yet another embodiment.

DETAILED DESCRIPTION

The following provides examples of fabrication methods according toembodiments of the invention, which result in substrates of relativelylarge diameter while exhibiting low micropipes, screw and basal planedislocations densities. The apparatus and method employed are designedto exert no stress on the seed, so as to limit defects in the growncrystal. No clamping, bonding or mechanical attachments are used toretain the seed in place, and the seed is allowed to thermally expandwithout constraints, but the vertical movement is limited such that theback surface of the seed is prevented from contacting the surface of thelid. The following are specific embodiments illustrating how the seedcan be retained in place without physically attaching it to the lid,although other embodiments may be used to achieve the same effect.

According to the following embodiments, the process begins with an SiCseed, which is generally in the shape of a circular wafer of the samediameter as the grown crystal. The seed crystals are polished and oretched on the carbon face to ensure that the surface is free ofmechanical damage.

Embodiments of the method described herein provide several advantagesover prior methods and apparatus for seed crystal growth. Most prior artmethods connect the seed with contact between the back surface of theseed and the top of the crucible or reaction cell/lid. However, we havefound that such contact is unnecessary and may be detrimental to crystalformation. We have unexpectedly found that by supporting a seed crystalin a configuration in which the seed is contacted only at its periphery,the stresses imparted to the crystal during growth are minimized. Wehave also unexpectedly found an improvement in the quality of the growncrystal as evidenced by the low crystal defect density.

This configuration of the support minimizes damage to or bending of theseed crystal during the mounting process, and because the seed crystalis mechanically decoupled from the reaction cell, the seed crystal mayexpand and contract independently of the reaction cell during growth.

FIG. 1 shows a generic arrangement for growth of SiC crystals byphysical vapor transport, indicative of the prior art. PVT reaction cell40 having lid 43 is illustrated which is configured for SiC crystalgrowth. The reaction cell is typically formed from a graphite vessel.Granulized SiC or silicon and carbon material 42 is placed in the bottomof the cell. Positioned in the upper portion of the vessel is a seedcrystal 48 mounted to the inside of the top of the cell, e.g., clampedor bonded to lid 43. Notably, in the configuration of FIG. 1, duringprocessing the back surface of the seed 48 contacts the bottom surfaceof the lid 43, whether by physical attachment, such as adhesive orclamp, or by pressure differential, as explained in JP 2011-20860.

The entire vessel is surrounded with insulation 54 such as graphite feltor foam. The reaction cell 40 is placed in a vacuum furnace 70 which ispumped by a vacuum pump. The vacuum furnace 70 may be comprised of steelif the cell is resistively heated, or it may be comprised of glass ifthe cell is inductively heated. In the embodiments shown, the vacuumfurnace is comprised of glass and is heated by an RF induction coil 72.Silicon and carbon evaporate from the source material 42 and condenseonto the seed 48. Silicon and carbon that has not condensed onto theseed is diffused out of the reaction vessel and into the vacuum furnace.This diffusion is driven by pressure gradient between the interior ofthe reaction vessel and the vacuum furnace. Gases that are beinginjected into the vacuum furnace, such as nitrogen, argon, and dopants,diffuse through the graphite crucible walls and into the reactionvessel. This diffusion is driven by concentration gradient between thevacuum chamber and the interior of the reaction vessel.

Referring now to FIG. 2, a PVT reaction cell according to one embodimentof this invention is illustrated, which is configured for low-stress,low-defect, SiC crystal growth. The reaction cell 40 is preferablyformed from a graphite vessel and has lid 43. Granulized SiC or siliconand carbon material 42 is placed in the bottom of the cell. The cell hasan interior diameter designated as “d” in FIG. 2, which is equal to orlarger than the diameter of the seed 48. Additionally, a shelf 46 isprovided in the upper portion of the cell. The shelf 46 has an innerdiameter, designated “d_(s)” in FIG. 2, which is slightly smaller thanthe diameter of the seed 48. Of course, the walls of the shelf need notbe vertical, but instead may be slanted, as shown by the dotted line, inwhich case the diameter “d_(s)” can have a larger value on the sourceside of the shelf than on the seed side. The shelf 46 can be made as aring of graphite bonded to the sidewall of the vessel. Alternatively,the shelf 46 may be made integrally with the vessel, e.g., the vesselmay be formed with the shelf as integrated part of the interior sidewallof the vessel 40.

Shelf 46 is used for supporting the seed crystal 48 at its peripheraledge, without any physical attachment. Rather, cushion rings 50, 51 areplaced above and or below the seed, contacting the upper and lowersurface of the seed 48 at its periphery. The cushion rings allow theseed to flex and expand while keeping the seed in the desired locationrelative to the cell and prevent the seed from physically contacting thelid 43 throughout the growth process. The cushion rings may be made ofmaterial that is softer than SiC, is malleable, and can withstand theprocess conditions and chemical reactions associated with SiC crystalgrowth. For example, molybdenum or graphite can be used to fabricate thecushion rings 50 and 51. Optionally, a graphite retainer, 53, ispositioned above the upper cushion ring 50, so as to maintain the seed'svertical position and prevent the seed from contacting the lid. However,it should be appreciated that the upper ring 50 and retainer ring 53need not press on the seed, but rather should allow free movement of theseed in the horizontal direction, as illustrated by the double-headedarrow in the callout. That is, the bottom cushion ring 51 is placed onthe shelf 46, the seed 48 is then placed on top of the bottom cushionring 51 and the top cushion ring 50 is placed on top of the seed 48. Theretainer ring 53 is placed over the top cushion ring 50 and then the lid43 is attached to the cell 40. This overall arrangement allows the seedto be in a controlled vertical position, and yet be free to expand andcontract without creating stress in the seed upon heating and cooling.Finite element modeling shows that this geometrical arrangement toposition the seed will correspond to a maximum stress in a bulk SiCcrystal of less than 5×10⁶ dynes/cm².

FIG. 3 illustrates another embodiment for forming the shelf 46 asintegral part of the cell 40. Specifically, the cell 40 is fabricatedsuch that its interior diameter is slightly smaller than that of theseed 48, similar to diameter d_(s) in the embodiment of FIG. 2. Theupper part of the cell 40 is fabricated to have a diameter that isslightly larger than the diameter of seed 48, i.e., similar to diameterd in the embodiment of FIG. 2. The difference between the two diametersforms shelf 46, upon which the bottom cushion ring 51 rests. Thus, ascan be understood, various configuration can be used to facilitate theshelf 46, and the particular configuration used is not essential,provided that the seed can be placed with the cushion rings on theshelf, such that the seed can expand and contract freely without havinghorizontal physical constraints, while at the same time preventing theseed from physically contacting the surface of the lid 43.

The embodiment of FIG. 3 also illustrates making the retainer ring 53 asintegral part of the lid 43. Specifically, as illustrated in FIG. 3, thelid 43 is fabricated with a protruding ring 53, serving to limit thevertical motion of the cushion ring 50, and thereby limiting thevertical motion of the seed 48. The ring 53 may directly contact theperipheral edge of the seed 48, or it may contact the cushion ring 50,when cushion ring 50 is used.

FIG. 4 illustrates yet another embodiment for supporting seed 48 withoutimparting stresses and enabling free expansion and contraction.Specifically, the shelf 46 is fabricated close to the top opening of thecell 40, such that the retainer is not needed. Instead, the lid 43serves to restrict the vertical motion of the upper cushion ring 50 bycontacting the cushion ring 50, such that the seed cannot contact thesurface of the lid 43. Another feature shown in FIG. 4 is the uppercushion ring 50 being thicker than the bottom cushion ring 51. This isdone in order to keep the seed 48 sufficiently far from the lid 43 andreduce the thermal influence of the lid 43 on the seed 48.

The following description is applicable regardless of the specificembodiment utilized. The entire vessel is surrounded with insulation 54such as graphite felt or foam. The thickness, thermal conductivity, andporosity of the insulation are chosen so as to achieve a desiredtemperature distribution in the reaction cell. The arrangement forgrowth of SiC crystals can include a controller 80 for controlling avalve 82 to dopant gas, e.g., a nitrogen source 84, which is connectedto the vacuum furnace 70, and for controlling a valve 86 to an argonsource 88, also connected to the vacuum furnace 70. The vacuum insidevacuum furnace 70 is controlled by valve 90 leading to vacuum pump 92.In this embodiment, controller 80 is configured to control vacuum valve90 and pump 92 so as to maintain a user-set vacuum level inside thevacuum furnace 70, regardless of argon and/or nitrogen flow into thevacuum furnace. For example, if nitrogen flow into the chamber isincreased, controller 80 opens the vacuum valve 90 to increase vacuumpumping from the furnace and maintain the set vacuum level. Controller80 also controls the operation of the heater, such as the power appliedto RF induction coil 72.

Once the cell 40 is loaded with the source material 42 and seed 48, itis sealed and is placed into an RF induction furnace 70. The furnace isevacuated using pump 92, thereby creating a pressure differentialbetween the interior of the furnace 70 and the interior of the cell 40.However, cell 40 is constructed such that the lid does not seal the cellfully hermetically and so, gaseous matter from inside the cell 40 leaksto the interior of furnace 70 and is pumped out. Similarly, the walls ofcell 40 are somewhat porous to gases and leak into the interior offurnace 70. Consequently, the pumping action of pump 92 also evacuatesthe interior of cell 40 by creating the pressure differential betweenthe interior of cell 40 and the interior of furnace 70.

Once the interior of cell 40 and furnace 70 have been evacuated, theinterior of furnace 70 is backfilled with a non-oxidizing gas such asargon from argon source 88. Pressure is established near atmosphericpressure (500-700 torr) by controlling vacuum valve 90, and thecontainer is heated to approximately 1,600-2,500° C. by energizing coils72.

The pressure is subsequently reduced to initiate the vapor transportprocess. In this method, the pressure is first reduced to the range of10-100 torr. Nitrogen gas can be added to the furnace to control theconductivity of the grown crystal, but regardless of nitrogen flow, thecontroller maintains the pressure at the set value, i.e., in the rangeof 10-100 torr.

At this point, the pressure, temperature and nitrogen flow arecontrolled to achieve the conditions needed to form the bulk SiC crystalon the seed. The thickness of the remaining crystal grown is in therange of 5-50 mm. Typical values of pressure are in the range of0.1-10.0 torr and temperature in the range of 2,000-2,500° C.

At the end of the growth process, the pressure is raised toapproximately 600 torr. This suppresses any more vapor transport. Thefurnace is then cooled to room temperature. When the container isopened, a single crystal of SiC with the same polytype as the seedcrystal has formed on the seed crystal.

New seeds can be created by slicing the crystals grown by this method,and the new seeds can be used to grow new crystals. It is found thateach generation of crystals grown shows reduced dislocation density.

As can be appreciated from the above, the method according to thedisclosed embodiments involves holding a seed in a vapor transportreaction cell such that the stress imparted to the SiC crystal duringgrowth is minimized to less than 5×10⁶ dynes/cm² and, as such, themethod can be integrated into any strategy to grow large crystals, e.g.,having diameter larger than 76 mm and length longer than 25 mm.

To assess the dislocations in the crystal, the crystal is sliced and allslices are polished. Micropipes can be tested by first revealing themwith molten salt etching and counting via manual and automated methods,or by scanning the polished slice with a laser light scatteringspectrometer and an image processing algorithm to count the micropipes.Methods such as this are described in J. Wan, et. al., “A New Method ofMapping and Counting Micropipes in SiC Wafers” Proc. 2005 Int'l Conf. OnSiC and Related Materials, Materials Science Forum (2006), p. 447, andJ. Wan, et, al., “A Comparative Study of Micropipe Decoration andCounting in Conductive and Semi-Insulating Silicon Carbide Wafers,” J.Electronic Materials, Vol. 34 (10), p. 1342. Once the total number ofdefects is counted, this value is divided by the area of the slicemeasured to derive a defect density in counts per unit area.

Screw dislocations and basal plane dislocations require either moltensalt etching as described above or x-ray topography. Counting isgenerally done by sampling several areas on the slice and counting thedefects. The method typically used to report defects consists of ameasurement at the center of the wafer, four sites 90 degrees apart at50% of the wafer radius and four sites 90 degrees apart at >80% of thewafer radius, and rotated 45 degrees to the points at 50% of the radius.The total counts in each site are summed, and then the sum is divided bythe measurement area to derive a defect density in counts per unit area.Since the sampling method of larger wafers is important to assessing thewafer, it is often pertinent to report the site count values as well asthe net count of defects

Example

A graphite vessel was formed and loaded with a source mixture of siliconand carbon totaling approximately 800 grams. A 4H—SiC seed ofapproximately 70 mm diameter was fixtured into the cell per the methodsdescribed earlier with the C-face facing the source. The graphiteassembly was wrapped with graphite felt and placed into a RF inductionheated vacuum furnace. The vacuum chamber was evacuated to base pressureand then backfilled with argon gas. The pressure was set to 600 Torr andthe system was heated to achieve a temperature of approximately 2200° C.as read by a pyrometer focused on the lid of the graphite vessel. N2 gaswas delivered to the chamber at this time and the pressure was droppedbelow 10 Torr to initiate sublimation of the source. The N2 gas flow wasset to a level to deliver a nitrogen concentration in the crystal withinthe range 3-8E18/cm³. After about 100 hrs the pressure was raised to 600Torr to stop sublimation and then the system was cooled to retrieve thecrystal.

The resulting crystal, designated BH9013, was 19 mm long on the taperedside. Crystal was sliced into wafers which were offcut 4 degrees toward<11-20> and the wafers were polished using diamond abrasives to achievea smooth specular surface.

Resistivity measurements performed on the wafers shows the values in thecrystal ranged 0.016-0.026 ohm-cm.

One of the slices was etched in molten KOH to reveal the dislocations.Micropipes were measured by counting all the micropipes on the wafer anddividing by the wafer area. In this case 10 micropipes were identifiedand this corresponds to a micropipe density of 0.12/cm². Dislocationswere measured at 9 sites on the wafer arranged by radius and angle. Thetest locations consists of a measurement at the center of the wafer,four sites 90 degrees apart at 50% of the wafer radius and four sites 90degrees apart at >80% of the wafer radius, and rotated 45 degrees to thepoints at 50% of the radius. Microscope images were taken at each siteand the etch pit density ((EPD) which is the sum of all threading, basaland screw dislocations), basal plane dislocation density and screwdislocation density were determined from the images. The data istabulated below:

Basal Plane Dislocation Screw Dislocation Site EPD (cm-2) Density (cm-2)Density (cm-2) 1 1.25E+03 0 0 2 2.00E+04 500 0 3 2.50E+03 0 0 4 6.00E+03250 0 5 1.50E+03 250 500 6 5.00E+03 0 0 7 5.25E+03 500 0 8 4.50E+03 0 09 0.00E+00 0 0

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. The present invention has been described inrelation to particular examples, which are intended in all respects tobe illustrative rather than restrictive. Those skilled in the art willappreciate that many different combinations will be suitable forpracticing the present invention.

Moreover, other implementations of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. Various aspects and/orcomponents of the described embodiments may be used singly or in anycombination. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

1. A graphite container for use in a furnace for performing SiC crystalgrowth on a disk-shaped seed having a known diameter, comprising: agraphite cylindrical container having a sidewall and an open topconfigured for accepting a graphite lid; a cylindrical shelf formed onan upper part of the sidewall and having an interior diameter slightlysmaller than the diameter of the seed, the cylindrical shelf beingformed at a defined distance below the open top, thereby enablingplacing and supporting the seed thereupon; a graphite lid configured forforming a closure with the open top.
 2. The graphite container of claim1, wherein the cylindrical shelf is formed integrally with the sidewall.3. The graphite container of claim 1, wherein the cylindrical shelf isformed as a ring of graphite bonded to the sidewall.
 4. The graphitecontainer of claim 1, further comprising means for preventing the seedfrom contacting the lid.
 5. The graphite container of claim 1, furthercomprising at least one cushion ring having a diameter slightly smallerthan the diameter of the seed and configured for placement below orabove the seed.
 6. The graphite container of claim 5, wherein the atleast one cushion ring comprises molybdenum or graphite.
 7. The graphitecontainer of claim 6, further comprising a graphite retaining ringhaving an exterior diameter to fit within the interior diameter of thesidewall and having an interior diameter slightly smaller than thediameter of the seed, the retaining ring configured for placement abovethe seed so as to confine vertical movement of the seed inside thecontainer.
 8. The container of claim 1, wherein the lid comprises agraphite retaining ring extending from bottom surface of the lid.
 9. Thecontainer of claim 1, wherein the cylindrical shelf comprises verticalwalls.
 10. The container of claim 1, wherein the cylindrical shelfcomprises slanted walls.
 11. The graphite container of claim 1, furthercomprising an upper cushion ring and a bottom cushion ring and whereinthe upper cushion ring being thicker than the bottom cushion ring. 12.The container of claim 1, wherein the sidewall has a diameter largerthan the diameter of the seed.
 13. A system for forming an SiC crystal,the system comprising: a. a graphite cylindrical container having asidewall and an open top configured for accepting a graphite lid; acylindrical shelf formed on an upper part of the sidewall and having aninterior diameter slightly smaller than the diameter of the seed, thecylindrical shelf being formed at a defined distance below the open top,thereby enabling placing and supporting the seed thereupon; and agraphite lid configured for forming a closure with the open top; b. aheater for heating an induction furnace to a temperature from 2,000° C.to 2,500° C.; c. a pump for evacuating the induction furnace to apressure from 0.1 Torr to >600 Torr; and d. gas inlet for filling theinduction furnace with an inert gas.
 14. The system of claim 13, furthercomprising at least one cushion ring allowing the seed to flex andexpand while preventing the seed from contacting the lid;
 15. The systemof claim 14, wherein the cushion ring comprises a first cushion ringpositioned below the seed and a second cushion ring positioned above theseed.
 16. The system of claim 15, further comprising a graphiteretaining ring positioned above the second cushion ring.
 17. The systemof claim 14, wherein the lid comprises a graphite retaining ringextending from bottom surface of the lid.
 18. The system of claim 13,wherein the cylindrical shelf is formed integrally with the sidewall.19. The system of claim 13, wherein the sidewall has a diameter largerthan the diameter of the seed.
 20. The system of claim 13, wherein thecylindrical shelf comprises slanted walls.